Shoot in Tricky Light Conditions

ISO controls and white balance demystified; plus, a fix for map junkies.

Dave Johnson

Choosing the Right "Film" Speed

Ah, the imponderables of modern life! Why can you be overwhelmed, occasionally underwhelmed, but never just plain whelmed? Why do people think their dogs won't understand when a stranger speaks in a foreign language? And what do those ISO controls actually do on your digital camera? Should you use them, or just keep ignoring them?

First Some Theory

Your camera's ISO setting is simply a measure of how sensitive the camera is to light--in other words, the film speed, even though there's no real film in the camera. If you have a 35mm camera, you probably know that film comes in a wide variety of speeds--from ISO 25 all the way up to about ISO 1600. The lower the number, the slower the film will respond to light, requiring a longer exposure or larger aperture to properly expose the picture. Higher ISO numbers are more sensitive to light and consequently need less time to expose a picture.

How much do you need to adjust the exposure time for different ISO settings? Doubling the ISO number changes the exposure required by one step. So, if ISO 100 film would expose a picture for 1/125 second, ISO 200 film could take the same picture in 1/250 second. It's almost as if the film is competing in a photographic version of Name That Tune. "I can take that picture in...1/60 second!"

The Digital Angle

That's all well and good, but how does it apply to digital cameras? After all, there's no film in the camera.

Actually, the CCD, which is the light-sensitive chip that plays the role of the eye for most digital cameras, can be configured to be more or less sensitive to light. By default, most digital cameras use a sensitivity that's roughly equivalent to an ISO of about 64 or 100. But many cameras let you throttle that up to 200, 400, or sometimes even higher.

There are a few situations in which you might want to change the ISO setting. With a 35mm camera, you'd have to actually change the film to one with a different ISO rating. But it's a lot easier on a digital camera: You can just twist a dial or push a button. You can easily take pictures with different ISO values on the same memory card. Try that on a film camera!

When to Set ISO

When set to automatic, your camera's ISO is typically configured for its lowest setting, probably in the neighborhood of ISO 64 or 100. That's fine for most photography, but what if the light conditions are low? Frame your shot and see what shutter speed your camera wants to use. If the shutter speed is really slow--like 1/15 second or more--you can crank the ISO up a notch or two to bring the shutter speed into a more hand-holdable, jitter-free range.

You can use the same trick to avoid using a flash. I try not to use the flash because I don't like the hot, artificial look of flash photography. You might also be trying to shoot a subject that's just too far away for the flash. Remember, a digital camera flash generally only lights up about a dozen feet or so. By increasing the ISO setting, I can often take pictures indoors without turning the flash unit on.

Finally, if you're trying to be creative, changing the ISO setting can again come to the rescue. Suppose you want to take a picture with a long depth of field, so that the entire picture, foreground and background, is in focus. For that, you'd need a tiny aperture setting, like f16. But--you guessed it--it's too dark and the camera wants to open the aperture way up. The solution? Increase the ISO so you can close the aperture.

ISO Pitfalls

These are some reasons, in a nutshell, when and why you'd want to increase the ISO level of your digital camera. But beware: When you make the CCD more sensitive to light, you increase the odds of getting digital "noise" in your picture. That's why you should always step the ISO back down to its normal level when shooting in ordinary lighting. An ISO of 400 is too much for most digital cameras, and your photos will suffer.

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Dave's Favorites: Terraserver

I admit it--I'm a map fanatic. I like GPS navigation systems, paper atlases, compasses, and especially the ultimate marriage of maps and digital photography: satellite imagery. I love that a photo taken by a satellite 250 miles above our heads can capture details as small as just a few feet across.

You no longer have to work at the CIA to get a close look at satellite photography. Visit Microsoft's Terraserver for an up-close and personal look at virtually any site on earth.

Just enter the location you'd like to see, and the site displays the latest imagery of the area, sometimes giving you your choice of data. I especially like the way a small window displays a synchronized street-level map of the area, helping you to zoom in to find your old high school or your sweetheart's house. You can even download or print the images for free, or purchase prints and high-resolution data.

Q&A: What's the White Balance for?

I have a point-and-shoot digital camera with white balance choices. How do I use the white balance settings?

-- Tim Long, Elizabeth, New Jersey

The white balance control tells your digital camera which color in the picture should actually be exposed as "white." You can leave your camera's white balance selector set on automatic most of the time.

There are some situations in which you might want to adjust it, though. If you're shooting in tricky lighting conditions, such as in a gymnasium or by candlelight, choose the appropriate option from the menu and take your picture--but set it back to normal when you're done.

The white balance control can help you be creative, too. Varying the white balance can make the picture look warmer or cooler, and even give a sepia tinge to your photos.

Send your questions to question@bydavejohnson.com, and please be sure to let me know where you're from.

Hot Pic of the Week

Get published, get famous! Each week, we select our favorite reader-submitted photo based on creativity, originality, and technique. Every month, the best of the weekly winners gets a prize valued at between $10 and $100.

A gentle reminder, folks: We're disqualifying some really wonderful pictures every week because the submissions aren't following the rules. Be sure to include everything we ask for in the e-mail message, or your entry is wasted!

Here's how to enter:

Send us your photograph in JPG format, at a resolution no higher than 640 by 480 pixels, to hotpic@pcworld.com. Entries at higher resolutions will be disqualified immediately. If necessary, use an image editing program to reduce the file size of your image before e-mailing it to us. Include the title of your photo, along with a short description of the photo and how you photographed it. Don't forget to send your name, e-mail address, and postal address. Before entering please read the full description of the contest rules and regs.

This Week's Hot Pic:

Little One by Gary Farrell, Thunder Bay, Ontario

Gary says:

"Battered and bruised after a solid week of paddling canoes and kayaks in Quetico Park in Northwest Ontario, our little one strikes a pose silhouetted by yet another breathtaking sunset. Precambrian Shield boulders, worn smooth by ancient glaciers, wear coats of lichen and moss. The little one wears DEET, bug bites, and scabs. This was shot on our last night in the park with a Fuji Finepix 2400. I preset the exposure for the sky and used a fill-flash."